Generally, an automotive vehicle is driven by a single internal combustion engine such as a gasoline engine or a diesel engine, and the cylinder volume of the internal combustion engine is constant during operation. Since low specific fuel consumption of the internal combustion engine is limited within a small range which is decided by engine torque and engine speed, and since the engine for automotive vehicles is operated under wide driving conditions, the fuel consumption becomes unsatisfactory. In order to eliminate such a disadvantage, there is provided an internal combustion engine comprising a plurality of engine units, which are used in accordance with driving conditions of the vehicle for effectively generating power, whereby the fuel consumption may be improved. For example, Japanese patent publication No. 42-26050 discloses an internal combustion engine, in which a pair of engine units are disposed in parallel and the power of both units is transmitted to an output shaft. Each engine unit is controlled in accordance with load on the engine. However, in such a system it is difficult to use the engine units so as to improve fuel consumption in all conditions of the engine operation.